"You're inventing your own misery."

I'm sorry, what?
Why do people believe that telling someone to "stop being miserable and just be happy" will make things any better? What the fuck?
Right now, I'm battling through my revision and thinking how utterly useless it is since I'm a useless, worthless idiot who can't do anything right and so I will inevitably fail and revision is therefore a waste of time. I think like that because my thought process is nothing but self-loathing, regret and guilt.
Oh, of course.
My suicidal, self-hating thoughts will just go away if I 'stop being miserable'.
Please, do tell me.
How the fuck do I stop being miserable?
I don't want to be miserable. This isn't a goddamn fucking choice. I would give anything to be happy, but it's not as easy as that. I spend most of my time either at college or doing college work, and it's draining. I don't have time to go out and socialise and try and cheer myself up, because all the things I have to do are in the back of my mind.
And I can't just not worry, since my exams are basically deciding my future. If I fail, I'm fucked. Done. My uni place is gone and I've fucked up royally.

But of course, I'll just be  happy. That will make everything go away.

Fucking idiots. Don't talk shit about things you don't understand.